Excimer lasers are commonly used gas lasers oriented to ultraviolet applications. At present, the excimer lasers are considered as best candidates for light sources for photolithography, and are predominant working light sources in the Integrated Circuit (IC) photolithography industry.
Conventional discharge excited excimer lasers adopt a single-cavity single-electrode configuration. With the further development of the photolithography technology, the light sources are desired to meet requirements of narrower spectral width (line width), higher repetition rate, and greater average power. However, the conventional single-cavity single-electrode configuration can hardly meet all these three requirements at one time. This tends to cause a significant compromise between performance enhancement and cost efficiencies in laser designs. Difficulties in improvement of the single cavity configuration of the conventional lasers mainly consist in great energy losses in line narrowing modules and damages and life spans of optics under high-power laser radiation.
To effectively narrow the spectral width and enhance laser output power, a dual-cavity configuration was proposed to design a laser. A fundamental concept of this configuration consists in that narrowing of the line width and enhancement of the laser output power are achieved in different gas discharging modules (specifically, a seed cavity and an amplifier cavity), respectively. Specifically, the seed cavity generates seed light with a narrow line width at a certain repetition rate, to achieve laser oscillation and radiation at low power. The amplifier cavity performs pulse energy amplification on the incident seed light. Lasers based on the dual-cavity configuration have output characteristics such as control of narrowed spectrum and output of relatively high single-pulse energy, which are necessary for the light sources for photolithography.
The lasers based on the dual-cavity configuration can be increasingly optimized in both the master oscillator module and the amplifier module, by optimizing components and pressure of a working gas mixture, exciting voltages and the like, to enhance the output characteristics, so as to achieve laser outputs with a narrow line width at high power. Further, due to the power amplification mechanism based on the amplifier cavity, the master oscillator has a relatively low laser output, resulting in significantly increased life spans of optics in the line narrowing module. Because of the above advantages of the lasers based on the dual-cavity configuration, laser designs based on the “seed-amplification” mechanism are widely used in modern light sources for the photolithography industry.
The dual-cavity designs can be classified mainly into three types, i.e., a dual cavity Master Oscillator Power Amplifier (MOPA), a dual cavity Master Oscillator Power Oscillator (MOPO), and a dual cavity Master Oscillator Power Regenerative Amplifier (MOPRA) which is an improvement of MOPA. Structural details for these designs are shown in FIGS. 1, 2, and 3, respectively.
FIG. 1 is a structural view showing a dual-cavity MOPA excimer laser according to the related art. As shown in FIG. 1, the dual-cavity MOPA excimer laser comprises a Master Oscillator (MO) cavity 101, a Power Amplifier (PA) cavity 102, a Line Narrowing Module (LNM) 103, a Linewidth Analyzing Module (LAM) 104, a MO light-path conversion and control module (MO WEB) 105, a PA light-path conversion and control module (PA WEB) 106, an Optical Pulse Stretcher (OPS) 107, a Bandwidth Analysis Module (BAM) 108, a penta prism 109, and an auto shutter 110.
The MOPA configuration is a laser system design which was firstly used in advanced light sources for photolithography. This configuration was described in US 2002/0044586 A1, US 2006/0126697 A1, and U.S. Pat. No. 6,690,704 B2. As stated in the document, “Recent Developments in ArF Excimer Laser Technology for Photolithography,” pp. 523-524, in the MOPA configuration, the capability to amplify the laser energy is limited due to a limited number of passes of the laser through the amplifier cavity. As a result, the MO (Master Oscillator) cavity needs to output higher laser power to satisfy the requirements on the light sources. Specifically, the output from the MO cavity, after being line-width narrowed, should convey the seed light at power of about 1 mJ to the PA (Power Amplifier) cavity. The relatively great energy losses caused by the line-width narrowing mechanism results in a relatively low conversion efficiency. Discharging excitement at high power makes the MO cavity have a significantly lowered life span. Further, the output from the PA cavity is affected by a precision of synchronization of discharging between the MO cavity and the PA cavity. Thus, the stability of the laser energy output need to be further improved.
The above deficiencies of the MOPA configuration can be eliminated by the MOPO configuration based on the injection lock technology and the MOPRA configuration based on the recirculating ring technology.
FIG. 2 is a structural view showing a dual-cavity MOPO excimer laser according to the related art. As shown in FIG. 2, the dual-cavity MOPO excimer laser comprises a Power Oscillator (PO) cavity 201, a PA cavity 202, a LNM 203, and alight path system including a concave mirror 204 and a convex mirror 205.
US 2008/0285602 A1 discloses a design based on the MOPO dual-cavity configuration.
FIG. 3 is a structural view showing a dual-cavity MOPRA excimer laser according to the related art. As shown in FIG. 3, the dual-cavity MOPRA excimer laser is an improvement of the MOPA configuration, and is configured like MOPA, except that a PA web 306 and a BAM 308 are exchanged in position. Thus, the seed light can get a multi-pass gain.
US 2010/098120 A1 discloses a design based on the MOPRA ring cavity configuration.
In the MOPA configuration, the seed light gets only a limited number of multi-pass gains through the PA cavity, and thus the MO cavity needs to inject the seed light of about 1 mJ into the PA cavity to achieve the laser output of about 10 mJ. In the MOPO configuration based on the injection lock technology and the MOPRA configuration based on the recirculating ring technology, the amplifier cavity can achieve a multi-pass gain, instead of the limited number of multi-pass gains in the MOPA configuration. The PO cavity and the PRA (Power Regenerative Amplifier) cavity operate in an oscillation and amplification status, and the seed light gets a multi-pass gain. As a result, the seed light of only 100-200 μJ will result in the laser output of 15 mJ. The injection lock technology and the recirculating ring technology has a significant feature that the seed light, after being injected into the amplifier cavity, goes forward and backward and resonates in the amplifier cavity, and the amplifier cavity operates in a deep saturation status. They thus exhibit advantages such as greater energy and more stable output, as compared with the MOPA configuration.
The MOPA, MOPO, and MOPRA configurations are all based on a dual discharge cavity configuration. Lasers based on the dual-cavity configuration are more expensive, bulkier, and more complex in manufacture and manipulation, as compared with conventional lasers based on a single-cavity configuration. Specifically, the lasers based on the dual-cavity configuration put a stringent requirement on synchronization of discharging in order to achieve a good energy amplifying property, leading to difficulties in synchronizing discharging. In addition, the lasers based on the dual-cavity configuration are complex in structural aspects, resulting in increased difficulties in manipulation such as mounting and adjusting of peripheral components.